The present invention relates to a radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of the human head, whose geometry consists of a single end-ring connected to a plurality of legs which extend along a cylinder and are joined in pairs on a hemispherical dome.
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these and other references may be found at the end of the specification immediately preceding the claims. The disclosures of all of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
For the resonator described above the most homogeneous normal mode of this structure is doubly degenerate and affords quadrature operation. The high sensitivity in the hemispherical end is particularly suited to human brain studies. This resonator represents a clinical application of two-dimensional ladder network resonant structures whose operation may be understood by analogy to the mechanical problem of oscillating two-dimensional membranes.
Straightforward analysis of the resonant behavior of LC ladder networks may be accomplished by solving the eigenvalue problem defined by the Kirchoff mesh equations. The low-pass birdcage resonator (1) has been completely characterized using this technique (2,3). In some cases, an analogy to one-dimensional mechanical coupled mass-spring systems, where the mesh current amplitudes are analogous to the displacements of the masses, provides a more intuitive understanding of these networks (2). For example, the low-pass birdcage resonator is analogous to a one-dimensional coupled mass-spring system with periodic boundary conditions. The amplitudes of the resulting mesh currents vary sinusoidally with the mesh index and integral numbers of wavelengths are allowed (2). Similarly, a nine-leg half-birdcage resonator (4) has been shown to correspond to a coupled mass-spring system with fixed end conditions which result in sinusoidal distributions of mesh current amplitudes with half-integral multiples of the wavelength allowed.
Recently, the electro-mechanical analogy has been extended to finite length, two-dimensional LC ladder networks (5). Specifically, the correspondence between a 2D ladder network resonator and a mechanical vibrating membrane was exploited (6). By assuming independent resonant operation in each spatial dimension and applying appropriate boundary conditions, the resonant mode structures of square-mesh, planar 2D ladder networks were predicted (5).